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Exploring Substances: Acidic, Basic, and Neutral – Class 7 Science notes, important questions and MCQs

NCERT Class 7 Science, chapter 2. Notes, key terms, important questions with answers and MCQs.

Chapter overview

Substances can be acidic, basic or neutral. Indicators like litmus, turmeric and China rose show which is which by changing colour.

Acids and bases

Acids taste sour (lemon, vinegar, curd); bases taste bitter and feel soapy (soap, baking soda, lime water).
Never taste or touch unknown chemicals.
Neutral substances are neither acidic nor basic (pure water, sugar solution, salt solution).

Indicators

Litmus: acid turns blue litmus red; base turns red litmus blue.
Turmeric turns red with bases; China rose turns acids dark pink and bases green.
Neutralisation: acid + base → salt + water + heat (antacids for acidity; ant bite treated with baking soda).

Key terms

Indicator: substance that changes colour in acids or bases
Neutralisation: reaction between an acid and a base
Antacid: base that relieves excess stomach acid

Formulas

Acid + base → salt + water

Important questions with answers

Name two natural indicators.2 marks

Litmus (from lichens), turmeric, and the China rose (hibiscus) flower.

What is neutralisation? Give one everyday example.3 marks

The reaction between an acid and a base that gives salt and water, with heat released. Example: taking an antacid (milk of magnesia) neutralises extra stomach acid in indigestion.

Explain three uses of neutralisation in daily life and agriculture, and how you would test whether an unknown solution is acidic, basic or neutral.5 marks

Indigestion: antacids neutralise extra acid in the stomach. Soil treatment: acidic soil is treated with lime (base), and basic soil with organic matter, so crops grow well. Factory waste: acidic waste is neutralised with bases before release to protect water life. Testing: put drops on red and blue litmus. Blue turning red = acidic; red turning blue = basic; no change = neutral (China rose or turmeric indicators can confirm).

Quick revision

Acid: blue litmus → red.
Base: red litmus → blue.
Turmeric turns red with base.

Exam tips

Acidic soil is treated with lime (base); basic soil with organic matter.
Factory waste is neutralised before release.

Practice MCQs

Lemon juice is
1. acidic
2. basic
3. neutral
4. salty only
Show answer
Answer: (a) acidic. Citric acid.
A base turns red litmus
1. blue
2. red
3. green
4. colourless
Show answer
Answer: (a) blue. Blue.
Which indicator turns red in soap solution?
1. litmus (blue)
2. China rose
3. turmeric
4. none
Show answer
Answer: (c) turmeric. Turmeric with a base.
An ant bite contains
1. citric acid
2. formic acid
3. baking soda
4. salt
Show answer
Answer: (b) formic acid. Rub a mild base like baking soda.
Turmeric paper turns red in
1. soap solution
2. lemon juice
3. sugar solution
4. distilled water
Show answer
Answer: (a) soap solution. Turmeric is an indicator for bases.
A farmer's soil is too acidic. He should add
1. quicklime or slaked lime
2. vinegar
3. lemon juice
4. sugar
Show answer
Answer: (a) quicklime or slaked lime. Bases neutralise acidity.
Which solution will not change the colour of either litmus?
1. sugar solution
2. soap water
3. vinegar
4. lime water
Show answer
Answer: (a) sugar solution. Neutral.
An ant bite is treated with
1. baking soda paste
2. vinegar
3. lemon juice
4. salt water only
Show answer
Answer: (a) baking soda paste. Formic acid is neutralised by a mild base.

All chapters of Class 7 Science

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.